Light, nutrients and primary productivity in Lake Biwa: An evaluation of the current ecosystem situation

Abstract

Simple correlation and multiple regression analyses were performed to examine the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the analyses were estimated monthly or bimonthly during the growing season (April–November) in 1992, 1996 and 1997 with the 13C method. Elemental (C, N and P) contents of seston were used to assess nutrient conditions. Analyses revealed that 86% of variance in depth‐integrated primary production rates (areal PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To assess relative effects of light and nutrients on PP, the P:B ratio was estimated by normalizing PP with sestonic C. The areal P:B ratio correlated most significantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correlated significantly only with the sestonic N:P ratio at 0 and 1 m depths, while light intensity was also incorporated into the regressions at deeper than 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results suggest that the primary production rate in this lake was mainly limited by P relative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as well as the nutrient supply seem to be important in determining the growth and composition of primary producers, since the surface mixing layer extends into 10–15 m depths during most of the growing season.

Long‐term changes in the abundance of Jesogammarus annandalei (Tattersall) in Lake Biwa

“…However, after 1970, total phosphorus in the surface layer of this lake tended to decrease, probably because of legal regulation of nutrient loading and the development of sewerage systems in the surrounding areas. In Lake Biwa, algal growth and primary production are limited mainly by the P supply (Urabe et al 1999). Therefore, it is unlikely that primary production, and, thus, the sinking flux of organic matter to the lake bottom, increased suddenly in 1985.…”

“…However, we could not ascribe the increase in abundance of J. annandalei to an overall increase in supply of their food. It is well known for a variety of organisms that the maturation size decreases with decreasing food supply (Urabe 1991;Johnson and Widerholm 1992;Urabe and Sterner 2001). Thus, the maturation size of J. annandalei suggests that per capita food supply was kept low after 1985.…”

Estimation of the fate of dissolved DNA in thermally stratified lake water from the stability of exogenous plasmid DNA

“…This phosphorus mineralization would be necessary for the growth of microorganisms, which are frequently limited by inorganic phosphorus (Toolan et al 1991, Coveney & Wetzel 1992. Several researchers have reported that phosphorus is an important growth-limiting factor for microorganisms, especially for phytoplankton, in the epilimnion of Lake Biwa, during stratification periods (Tezuka 1984, 1985, Frenette et al 1996, Urabe et al 1999). During our sampling period, a concentration of 23.8 µg l -1 dDNA was detected in the epilimnion.…”

Nutrient-Rich Microhabitats within Biofilms Are Synchronized with the External Environment

“…The respiration to photosynthesis ratio from spring to autumn has been reported to be in the range of 1.2 to 2.5 for Lake Superior (26), indicating a gross heterotrophic condition. In Lake Biwa, a positive relationship was noted for temperature and light intensity with regard to the primary production rate (31,33). However, temperature increases also lead to increases in heterotrophic microbes and their rates of respiration (34).…”